1. Field of the Invention
The present invention relates to a liquid-crys-talline mixture containing a liquid crystal composition and an azulenesquaric acid dye of the formula I ##STR2## where R.sup.1 is C.sub.1 -C.sub.12 -alkyl which may be substituted by halogen, amino, hydroxyl, C.sub.1 -C.sub.12 -alkoxy, phenyl, substituted phenyl, carboxyl, C.sub.1 -C.sub.12 -alkoxycarbonyl, C.sub.1 -C.sub.12 -haloalkoxycarbonyl, C.sub.1 -C.sub.12 -phenylalkoxycarbonyl, cyano, C.sub.1 -C.sub.12 -alkanoyloxy, benzoyloxy, substituted benzoyloxy and/or the radical ##STR3## where R.sup.6 and R.sup.7 are identical or different and are each independently of the other C.sub.1 -C.sub.4 -alkyl or phenyl, or is carboxyl, C.sub.1 -C.sub.12 -alkoxycarbonyl, where the alkyl chain may be interrupted by one or more oxygen atoms, carbamoyl, C.sub.1 -C.sub.4 -monoalkylcarbamoyl, C.sub.1 -C.sub.4 -dialkylcarbamoyl or the radical EQU L--O--CO--NH--R.sup.8
where L is C.sub.1 -C.sub.12 -alkylene which may be substituted by phenyl and R.sup.8 is C.sub.1 -C.sub.20 -alkyl, C.sub.5 -C.sub.7 -cycloalkyl or substituted or unsubstituted phenyl, and R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are identical or different and are each independently of the others hydrogen or C.sub.1 -C.sub.12 -alkyl which may be substituted by halogen, amino, C.sub.1 -C.sub.12 -alkoxy, phenyl, substituted phenyl, carboxyl, C.sub.1 -C.sub.12 -alkoxycarbonyl or cyano, with the proviso that when R.sup.5 is hydrogen the positions of the substituents CH.sub.2 -R.sup.1 and R.sup.4 within an azulene ring can also be interchanged for either or both azulene rings and that R.sup.1 is also hydrogen when R.sup.5 is methyl or ethyl which may each be substituted by phenyl or substituted phenyl or is C.sub.3 -C.sub.12 -alkyl, or when at least one of the two radicals R.sup.2 and R.sup.4 is phenyl or substituted phenyl.
2. Description of the Prior Art
The present invention further relates to the use of the abovementioned liquid-crystalline mixture for fabricating liquid crystal displays.
Liquid-crystalline materials which contain pleochroic dyes are known. Such materials find application in displays. The fundamentals of this subject are described for example in the Handbook of Liquid Crystals, p. 611 ff (1980), Mol. Cryst. Liq. Cryst. 55, (1979), 51, Electronik 14, (1982), 66, and the references cited therein.
Liquid crystal displays (LCDs) are also known. They include the high-resolution static picture LCD wherein laser beams are modulated by a signal, for example a video signal, of a visual image to be repro-duced and are deflected on to a liquid crystal cell in order to write the visual image to be reproduced into it. Thereafter the visual image can be projected onto a screen by irradiation with light.
In such a display, the liquid crystal mixture is normally present in the smectic phase which on irradiation with a laser gives phase transitions into the nematic phase and finally into the isotropic phase.
If the hot part of the liquid crystal following irradation with the laser is quenched following irradiation with the laser, the molecules of this part do not become reoriented but form a disordered texture. This intensively scattering texture is utilized in writing. The process of writing goes hand in hand with the production of this intensively scattering texture in the liquid crystal matrix in the form of spots of small diameter. The added dye brings about better resolution and increases the absorption of light energy. The written image can be erased by reconverting the texture into its original form, ie. the uniformly ordered smectic phase. This is done for example by applying an electric field or by heating.
If such a liquid crystal display, is to be used to store up-to-date images, it is necessary for the rewriting of the static picture to take place rapidly. This in turn requires the efficiency of the temperature transfer to be increased. This is done in general by ensuring optimal absorption of the writing laser beam in the liquid crystal cell.
Consequently, a liquid crystal cell of this kind should contain dyes which
have an absorption maximum in the wavelength range of a semiconductor laser beam, ie. within the range from about 700 to 900 nm, PA0 possess a sufficiently high absorption maximum, PA0 are substantially transmitting in the visible wavelength range, PA0 are soluble in the liquid crystal and PA0 have the property of dichroism.
EP-A-196,919 and DE-A-3,415,638 have already disclosed liquid-crystalline mixtures which contain azulenesquaric acid dyes.
However, it has been found that the dyes described therein are not sufficiently soluble.